Thermoplastic film or sheet material has achieved widespread use in the fabrication of many useful articles. Particular utility for thermoplastic film has been found in the area of covering materials, such as diaper film and operating room table covers, due to the moisture barrier character of the plastic film, among other properties. Thin film of this type is typically produced by extrusion, the two general methods of extruding film being blown tubing and slot-die extrusion.
In the slot-die method of producing thermoplastic film, the heated plastic extrudate or melt, typically composed of polyethylene, polypropylene, and copolymers thereof, is extruded out of a narrow slot in an elongated die. Flat film is generally made in considerable widths on relatively large machines which run at high production rates on the order of 300 to 1000 or more feet-per-minute. The hot extrudate extruded in the form of a web from the slot-die is drawn and stretched through an air-gap to the desired gauge thickness and width by stretching rollers which form a nip for this purpose. The nipping rollers may also serve as an embossing or calendering means for the film if particular textures or surface characteristics and properties are desired for the film. The film then typically passes from the nipping rollers to a chill roller where it is cooled and set, and then to a take-up roller.
It has been known to use linear low density polyethylene as the polymer composition in the slot-die extrusion of film. The densities of this polymer have varied usually between about 0.915-0.940 g/cm.sup.3. Linear low density polyethylene (herein simply sometimes LLDPE) as the term is used in this description is polyethylene having little or no long chain branching, with short chain branching in an amount necessary to obtain the desired density, currently for instance less than about 0.940 g/cm.sup.3. For example, a common density for LLDPE used in diaper film is 0.926 g/cm.sup.3, and for operating room table covers is 0.918 g/cm.sup.3. Linear low density polyethylene has been considered desirable in that it exhibits a good balance of mechanical properties in film form, particularly ultimate tensile strength, impact resistance, burst strength and tear strength characteristics.
A serious drawback to the use of LLDPE is its susceptibility to draw resonance at a relatively low draw or take-up rate, typically at a take-up rate of about 40 feet per minute or more. Draw resonance is a phenomenon associated with stretching motions and it is manifested in the occurrence of periodic fluctuations in the thickness of the extrudate as it is drawn. Draw resonance is primarily a function of the take-up rate as well as the draw ratio, the draw ratio being the ratio of the die-slot width to the film thickness. In regard to the latter, for a given linear low density polyethylene extrudate, draw resonance is expected to be more severe in drawing a thinner gauge of the film, and its onset at a lower draw speed, than in a thicker gauge film.
Film exhibiting draw resonance can have gauge thickness variations on the order of .+-.5%, or more, which is considered to be commercially unsatisfactory. Such gauge variations deteriorate product uniformity and reduce the reliability and saleability of the film. As a consequence, LLDPE which has been produced by conventional slot-die extrusion techniques has been drawn at take-up rates less than the critical point at which draw resonance sets in which is usually about a production speed of 30 to 40 feet-per-minute. This is a factor which seriously limits productivity.
There is consequently a need in the industry for a method for producing LLDPE film by slot-die extrusion techniques at relatively high draw rates with substantial elimination of draw resonance in the extrudate to thereby yield a thermoplastic film which is uniform in gauge thickness within the range of commercial acceptability.